Starch is an inexpensive, biodegradable, renewable polymer which has potential in many thermoplastic applications if means can be found to convert it into a melt-processable material. It is known that the starch granules can be thermally destructured or gelatinized by the effect of shear, heat and water to produce a thermoplastic material, which may further be plasticized with glycerine. The resulting material, perhaps because its molecular weight has been drastically lowered, is rather brittle, contains varying amounts of water which effects both the initial and the equilibrium properties, and the plasticizer may exude.
George et al., Poly. Eng. Sci., 34, 17-23 (1994) have described utilization of high levels of ethylene-vinyl alcohol copolymers in combination with starch to yield a more injection-moldable and more ductile blend. Lay et al., U.S. Pat. No. 5,095,054 describe many polymers useful in modifying starch; among theses are ethylene/acrylic acid copolymers, but such polymers must be neutralized to be effective. Further, Lay et al. teach that acid-containing polymers can have no other functionalized copolymers, including (meth)acrylic esters; thus, they specifically exclude the additive polymers of the present invention. They do teach polymers of vinyl pyrrolidone, but not in combination with (meth)acrylic esters and unsaturated acids. LaFleur et al., U.S. Pat. Nos. 5, 189,097 and 5,258,250 have shown that (meth)acrylic copolymers containing at least one of a vinyl amide or a carboxylic acid are useful in making poly(vinyl alcohol) melt-processable. A recent filing by some of the present inventors (European Unexamined Publication 583,109) discloses modification of starch into a thermoplastic material by utilizing the combination of poly(vinyl alcohol) and the (meth)acrylic copolymers of LaFleur et al.
None of the above approaches are totally satisfactory in converting starch to a thermoplastic material; they either require a combination of additives, often requiring a plasticizer, or they require high amounts of the additive polymer(s), or the starch still requires excessive processing with resulting degradation of molecular weight to make the starch thermoplastic.